The invention relates to a gas scrubbing tower usable in a plant for purifying the off-gases from industrial furnaces, especially from waste incineration plants which scrubbing tower has several superimposed stages, in which acid, neutral and basic harmful substances (pollutants) in the gaseous form or in the form of a mist, are still present in the waste-gases after prior withdrawal of thermal energy and after separation of at least a substantial portion or all of the solid pollutants, by means of preliminary treatments. Gaseous pollutants as well as the remaining solid pollutants and pollutant mists (aerosols), present in the gas are removed in the scrubbing tower by means of wash liquid which is then cycled through a sludge separator or thickener, from which pollutants suspension can be withdrawn.
Preferably, the thermal energy is withdrawn from the waste gases prior to their introduction in the gas purification plant in an indirect heat exchanger, in particular a steam generator being indirectly heated by the off-gases, before these enter the scrubbing tower.
A plant of the above type for carrying out the purification of off-gases from industrial furnaces for the treatment of waste-gases, is describe in Austrian Pat. No. 225,163 and in German Auslegeschrift No. 2,408,223, both of Gottfried Bischoff Bau kompl. Gasreinigungs- und Wasserruckkuhlanlagen KG, Essen, Germany.
The off-gases from waste incineration plants often have a high content of hydrogen chloride, stemming in particular from the combustion of polyvinyl chloride wastes, and, in addition, in particular a high content of SO.sub.2. Moreover, their content of primary dust is usually high.
German Auslegeschrift No. 2,431,130 of Walther und Cie AG, Cologne, Germany describes a scrubber which operates in principle with alkaline wash liquids, especially those containing sodium ions and ammonium ions, whereby concentrated salt solutions are obtained in the scrubber and these are recycled in the gas purification plant.
In a further known plant according to U.S. Pat. No. 3,929,963 of S.I. Taub (E.I. DuPont de Nemours and Company) the solid pollutants are removed from the waste-gas as completely as possible, by which means the need for a slurry separator can be avoided, whereupon the waste-gases, which have been substantially free from solid pollutants, are then subjected to scrubbing by means of a wash solution, the pH value of which in the scrubber or in the return line from the scrubber to the evaporative cooler can be so adjusted, by adding salt-forming chemicals, that a water-soluble or water-insoluble salt is formed with the gaseous acids or with other, basic pollutants taken up by the wash liquid.
Gas scrubbing towers containing several packing layers which extend each transversely to the perpendicular tower axis, and which layers are spacedly superimposed one above the other, in which towers the waste gas is introduced beneath the lowermost packing layer and the purified gas exists at or near the top end of the tower, have been described in numerous publications, for instance in
U.S. Pat. No. 2,523,441 to Paul E. McKamy issued Sept. 26, 1950,
French Pat. No. 1,469,230 to Metallgesellschaft, A.G. published Feb. 10, 1969.
U.S. Pat. No. 3,768,234 to Leslie C. Hardison, issued Oct. 30, 1973
German Offenlegungsschrift No. 26 43 211 to Gewerkschaft Keramchemie, publ. Apr. 6, 1978, and others.
Each packing layer consists of a large number of packing elements. "Hedgehog" type bodies as illustrated in FIG. 3 of a special printing of "Probleme der nass-mechanischen Abscheidung feiner Partikeln aus Gasstromen" taken from "Chemische Rundschau" 20(1975) No. 18 published by Chemie-Verlag Vogt-Schild AG, Solothurm, Switzerland are preferred.
More advanced scrubbing tower installations comprise at least one of the flue gas washing towers of the type described by Fattinger, Schmitz and Schneider in Publication No. 107 "Technik der Abgasreinigung" ("Technology of Off-gas Purification") of the "Tagung Lufthygiene 1976" ("Conference on Air Hygiene 1976") of Dec. 3, 1976 published by Verein zur Forderung der Wasser- und Lufthygiene (VFWL) of Huttenstrasse 36, 8006 Zurich, Switzerland (see FIGS. 1 to 4 and 10 therein).
A particularly advantageous tower of this type having a combination of a packing layer and an X-slot-separator is described in FIG. 10 of the last-cited publication.
These known towers suffer from a number of drawbacks which increase the cost of operation of the plants and increase the consumption costs for chemicals to an economically highly undesirable extent.
Thus, in all of these known plants, there is a sump at the bottom of the tower in which wash liquor charged with solid particles and/or scum collects, and often this wash liquor is then discharged into a settling tank and may be circulated from there to a sludge filter and back into the same settling tank. Supernatant liquor is then drawn off from the top sidewall of the settling tank and recirculated into one or several wash stages in the scrubbing tower.
The total amount of water taken up by the stream of waste-gas in the scrubber firstly results in a considerable increase in the volume of as to be purified and thus in a substantial, expensive increase in the size of the scrubbing tower and, secondly, in the formation of an undesirably intense plume of condensed water vapour at the outlet of the chimney from which the purified off-gases are released into the surrounding air.
Whilst the main object of the scrubbing tower is, in particular, to remove gaseous pollutants such as HCl, HBr, H.sub.2 F.sub.2, Cl.sub.2, Br.sub.2 and SO.sub.2 and also sulphuric acid mists from the off-gas, a further important function of the scrubber is to cool the gases, whereby intially gaseous tar-like substances and also salt-like substances can be condensed and then precipitated.
However, this treatment frequently causes precipitates on the walls of the various washing stages in the scrubber and also in the sludge separator, which precipitates form crusts or caking on these walls and in the outlets from the scrubber and impair the functioning of the entire washing apparatus to an increasing degree the longer the apparatus is in operation. These incrustations or caking consist, for example, of gypsum, lime, metal salts and other solids.